The previous series introduced warnings to multiple places, but it
could become tiring to see the warning on the same path over and
over again during a single run of Git. Making just one function
responsible for issuing this warning, we could later choose to keep
track of which paths we issued a warning (it would involve a hash
table of paths after running them through real_path() or something)
in order to reduce noise.
Right now we do not know if the noise reduction is necessary, but it
still would be a good code reduction/sharing anyway.